1. Field of the Invention
The invention relates to the field of pulverized fuel burners, particularly pulverized coal burners.
2. Prior Art
Recently, a trend has begun towards employing more coal, particularly for utilities. This trend has been brought about because of the abundance of coal, particularly in the United States, and current problems in supplying ample quantities of natural gas and oil.
Because of the present day recognition of pollution hazards, large consumers of fossil fuels are required to meet specified emission standards. among the pollutants which must be controlled are oxides of nitrogen, such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2), collectively referred to as nitric oxides (NO.sub.x). The nitric oxides are a strong public concern since in the form of NO.sub.2 they produce a visible brownish gas obscuring visibility, and combining with reactive hydrocarbons to form photochemical "smog" and/or possibly carcinogenic nitrites and nitrates.
Some attention has been given to the reduction of NO.sub.x in coal-fired furnaces. In U.S. Pat. No. 3,788,796 a pulverized fuel burner is described which provides means for reducing NO.sub.x. By maintaining a fuel-rich core, surrounded by combustion air, for a considerable distance from the burner, formation of NO.sub.x is reduced. Although this device provides satisfactory operation, it is complex, requires regular maintenance, and is not adaptable for existing burners, but rather requires an entire new burner.
In contrast, as will be described in detail, the improved burner of the present invention may be readily installed on existing burners, with minimum expense. Aerodynamic spoilers are employed on the primary air and fuel line to produce separate fuel-rich and air-rich zones.
In other prior art, vanes, and the like, have been employed primarily for providing efficient, thorough mixing of the pulverized coal and air. For example, in U.S. Pat. No. 2,755,750 a plurality of whirlers each of which includes fixed vanes, are used to thoroughly mix air, fuel and burnt gases. In contrast, the presently disclosed apparatus does not attempt to provide a uniform mixing, but rather to create fuel-rich and fuel-poor regions for the purpose of achieving NO.sub.x control. Nitric oxide tends to form fastest in the hottest (adiabatic) regions of the flames; i.e. the near burner region before boiler heat removal. If these adiabatic regions are maintained fuel-rich in a controlled manner then the scarce oxygen will selectively combust the fuel rather than attack the nitrogen in the air. In a similar manner chemically bound fuel nitrogen will tend to decompose or react to N.sub.2 in these same adiabatic, fuel-rich regions.
Fuel-rich/air-rich operation is currently employed in multi-burner natural gas and oil furnaces to reduce NO.sub.x. In such operation some of the burners are fed with air only (no fuel) or air is introduced above the flame zone or at other places so that the operating burners are fuel-rich (insufficient air); this provides off-stoichiometric firing. In coal-fired furnaces the use of off-stoichiometric firing or "over-fire air ports" can result in severe slagging of the lower boiler walls due to the reduction in the ash fusion temperature when moving from an air-rich (oxidizing) region to a fuel-rich (reducing) region. With the presently described apparatus unusual slagging does not occur since the air/fuel and combustion products finally obtain an air-rich condition before impinging the boiler walls (i.e. only the high temperature adiabatic initial zone is made fuel-rich for lowered nitric oxide emissions).